The Ehrlichia are obligate intracellular pathogens that infect circulating lymphocytes in mammalian hosts. Ehrlichia canis and Ehrlichia chaffeensis are members of the same sub-genus group that infect canines and humans and can each cause canine monocytic ehrlichiosis (CME) and human monocytic ehrlichiosis (HME), respectively. The canine disease is characterized by fever, lymphadenopathy, weight loss, and pancytopenia. In humans the disease is characterized by fever, headache, myalgia, and leukopenia. Early detection and treatment are important for treating both canine and human ehrlichiosis.
Indirect immunofluorescence assays (IFA) and enzyme-linked immunosorbent assays (ELISA) are frequently used as aids in the diagnosis of these diseases. These assays measure or otherwise detect the binding of anti-Ehrlichia antibodies from a patient's blood, plasma, or serum to infected cells, cell lysates, or purified Ehrlichia proteins. However, many assays for detecting anti-Ehrlichia chaffeensis antibodies or fragments thereof are severely limited in usefulness because of sensitivity and specificity issues directly related to the impure nature of the Ehrlichia antigen used in these tests. Additionally, animals vaccinated for E. canis may show a positive result when tested for E. chaffeensis due to immunological cross-reaction. Highly purified, specific reagents are needed to construct more accurate assays.